1 Answer
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Obviously since there's only 31 pages of D&D Next rules, these are the kinds of interpretations that have to be informed by earlier editions and DM fiat.

Usually "there's a bugbear in the trees" is not sufficient to avoid a sneak attack because the others still don't know exactly where it is. It certainly might allow them a second Wisdom check to spot it if he's pointing over in the general direction, perhaps even with Advantage if he can be a little more specific (and take an action to point it out).

The bushwhack damage would apply to anyone that hasn't spotted the bugbear by the time it attacks. Notice the power doesn't' say "unaware of his existence," it says "hidden from them."

In terms of who spotted him, I would say it depends how obvious the PC is being. If he spots the bugbear but keeps his peace, then the bugbear might mistakenly attack him and get more than he planned for - but if he's yelling "there's a bugbear!" and gesticulating in his direction, then no duh he knows that guy saw him.

5e is going back in the direction of earlier editions, where "the DM makes a ruling that seems like it makes sense in the game world" is always the correct answer, and there will not always be rules to cover every edge case.

I would say that it very much depends on how far the bugbears are. If more than 50% of their movement, then if they attack it won't be a surprise anyway. So if the character who saw the bugbears says: stop! It would anyway prevent the bugbear from attacking by surprise. If the characters are very close, then whatever the character who saw the bugbear does, the attack can happen and surprised characters receive the extra damage if hit.
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Alexis WilkeFeb 5 at 1:20